As Alley knows better than anyone, there are many factors that control how quickly a glacier can slide into the sea, including the amount of friction on the land it is sliding through, as well as how tightly it is buttressed by ice shelves. But another big issue is the strength of the ice itself. There are many differences between the Jakobshavn Glacier and Thwaites. For one thing, Thwaites is many times larger. The calving face of Jakobshavn is only about 10 miles long, versus 90 miles at Thwaites. Also, Thwaites is not constrained in a valley the way that Jakobshavn is, which means there is little friction on the sides to slow it down. If it really gets going, it could collapse much faster than Jakobshavn. More important, Jakobshavn does not sit on the edge of a reverse-slope basin the way Thwaites does. It can calve fast, but it is not what scientists call a threshold system. Thwaites is. But one thing they do have in common is that their structural integrity – and possible future collapse – is dictated by the basic physics of ice.

Standing 300 feet tall, the ice cliffs on the calving face of Jakobshavn are the highest anywhere on the planet. As it happens, there’s good reason for that. Alley and other scientists found that ice cliffs on marine-terminating glaciers like Jakobshavn or Thwaites have a structural limit of about 300 feet – after that, they collapse because of stress and weight. So, even if there are sections on Thwaites that are 6,000 feet deep, Alley realized, the structural integrity of ice would never allow a glacier’s face to stand that tall. In other words, glaciers with a face up to 300 feet can be relatively stable; after that, forget it. As Alley puts it to me, “It’s just collapse, collapse, collapse.”

One day, Alley was thinking about a problem that Dave Pollard, a colleague at Penn State, and Rob DeConto, a climate scientist at the University of Massachusetts, Amherst, had been having with their climate model. DeConto and Pollard had been collaborating for years to develop a sophisticated model to help them understand the impact of warming from fossil-fuel pollution on Greenland and Antarctica. Climate models are computer programs that try to capture fundamental physics of the natural world, such as, if the temperature warms one degree, how much will the seas around the world rise? It is not a simple question, and requires calculating everything from changes in how much sunlight the ice reflects to how much one degree of heat causes the Atlantic Ocean to expand. Models have gotten a lot better in the past few decades, but they still can’t simulate all the processes in the real world.

One way that scientists test how well a model might predict the future is by seeing how well it recreates the past. If you can run a model backward and it gets things right, then you can run it forward and trust that the results might be accurate. For years, DeConto and Pollard have been trying to get their model to re-create the Pliocene, the era 3 million years ago when the CO2 levels in the atmosphere were very close to what they are today, except the seas were 20 feet higher. But no matter what knobs they turned, they couldn’t get their model to melt the ice sheets fast enough to replicate what the geological record told them had happened. “We knew something was missing from the dynamics of our model,” DeConto tells me.

Alley suggested they plug in his new understanding of ice physics, including the structural integrity of the ice itself (or lack thereof), and “see what happens.” They did, and lo, their model worked. They were able to get the Pliocene melt just about right. In effect, they found the missing mechanism. Their model was now road-tested for accuracy.

The results were dramatic. The new processes combined to have a huge impact. Instead of about 2 meters of sea level rise, Antarctica lost enough ice to raise global sea level 17 meters over several thousand years. The fragile West Antarctic Ice Sheet collapses in a matter of decades, rather than centuries or millennia. There’s 5 meters of sea level rise in the first two centuries, after which retreat in portions of the East Antarctic Ice Sheet really get going.

Much more work will be required to make sure these new processes are being simulated accurately, but the early returns show it could put researchers in the ballpark of solving the puzzle of past high sea levels.

The relevance for our present situation is less direct, as the warming in the simulation was not realistic, but the possibility that West Antarctica could lose ice faster than we thought is a serious one. Richard Alley, whose work on this possibility we’ve covered before, explained to Ars via email, “I believe (and I suspect many people do) that it is important for us as scientists to provide not only the most-likely future outcome, but also the range of possibilities, including some sort of assessment of best-case and worst-case outcomes. Best-case is fairly easy, I believe, but worst-case is not; however, providing both is likely to be useful to many people.”

Flooding caused by sea level rise and other factors have affected human communities far more recently. Here in Cornwall there is the example of the Isles of Scilly (Note, you can use Scillonia but never the Scilly Isles). Virtually the entire central sound used to be inhabited perhaps as late as 100 CE and remained salt marsh untill about 300 – 400 CE

most major cities are located on coasts.
also, most of the world’s most productive agricultural regions are low lying, think Nile and Mekong delta.
If people have no place to live, no food to eat, they will move.
Current political turmoil in mideast and Europe in part a product of just a few million people on the move.

Having seen what is happening in Miami, Bangladesh, Virginia and the East Coast (US), it is becoming very, very unsettling as to what residents can do, especially in the future.

Collins Ave in Miami has a foot of water on it at high tides. Shorecrest is inundated regularly with septics discharging into the salt water flooding.

The most frightening aspect of this is that it’s an attitude of “Oh, well, we’ll adapt.” To the tune of hundreds of millions of dollars. The causeways to the Carolinas outer islands are covered in saltwater at king tides. Brooklyn is experiencing flooding. Virginia shore neighborhoods have streets awash at high tides.

What is particularly galling is this: THIS IS ONLY THE BEGINNING.

Our government and leaders are soundly asleep at the switch.

Add to this the increase in wildfires, exacerbated droughts and floods (Lake Powell, fed by the Colorado River is HALF EMPTY.) and the future looks very uncertain.

We are messing with planetary changes that will easily whelm us and very likely overwhelm us.

To the deniers & skeptics: What if you are wrong?? Will you indemnify us? Not bloody likely is it? Coal, oil and gas is not doing anything of note as regards the flooding above.

I do know this won’t affect me as much as it will my kids, grand-kids and their offspring. Who will compensate them for a huge encroaching struggle and loss?

We must all do our part to fight this pernicious laissez-faire attitude.